The operation and control of power systems are increasingly dependent on the Supervisory Control and Data Acquisition (SCADA) system. By attacking SCADA, an attacker can manipulate the information or reconfigure the device parameters to trip the circuit breaker. This paper considers several common ways to attack SCADA to trip the circuit breaker, and uses the attack tree model to evaluate the success rate of different attack scenarios. On the basis of the conventional reliability model of the circuit breaker, and considering the influence of SCADA cyber attacks, a revised forced outage rate model of the circuit breaker is established. Non-sequential Monte Carlo method is used to calculate power system reliability index. IEEE-RTS79 system simulation is used to analyze the influence of different attack scenarios on the reliability of power system, which verifies the feasibility and validity of the method. This work is supported by National High-tech R & D Program of China (863 Program) (No. 2015AA050104).